A number of learning and neurodevelopmental disorders are associated with reversed or anomolous neuroanatomicai asymmetries. One proposed manifestation of reversed or anomalous brain dominance is the inability to discriminate left from right. The purpose of the proposed studies is to evaluate the relationship between brain asymmetry and the discrimination of left and right from a phylogenetic perspective. It has been proposed that discrimination of left form right requires an asymmetric brain and that this ability is unique to humans. To test this theory, left-right discrimination will be assessed in monkeys and chimpanzees using conditional discrimination and matching-to-sample paradigms, and subsequently correlated with measures of neuroanatomical asymmetries and corpus callosum morphology taken from magnetic resonance images. Neurophysiological measures of interhemispheric transfer of information will be made to evaluate whether species differences in corpus callosum morphology are linked to differential callosal transfer time and fiber composition. The ability of monkeys and chimpanzees to learn to discriminate their left and right appendages in response to visual and acoustic cues and their ability to reference left and right from their own and another's perspective will also be examined. The spontaneous ability to use references points of left and right when learning visual imitation tasks will be assessed in each species. It is hypothesized that species and individuals with less asymmetric brains will perform significantly poorer on these tasks. Lastly, positron emission tomography (PET) will be used to evaluate the neural substrates of left-right discrimination in monkeys and chimpanzees. In this series of experiments PET scans will be collected while subjects are required to solve discrimination and imitation tasks requiring spatial transformation on the left-right compared to the top-bottom plane.